This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. The cyanobacterium Prochlorococcus is the numerically dominant photosynthetic organism in much of the world?s oceans;it is responsible for a large fraction of the world?s carbon fixation. To facilitate CO2 capture, Prochlorococcus, like all cyanobacteria, relies on a carbon concentrating mechanism that consists of the active uptake of inorganic carbon, intracellular accumulation and conversion to organic carbon within a primitive organelle, the carboxysome. Carboxysomes consist of self-assembling protein shells that encapsulate putative enzymes and other proteins that are functionally related. The shell of carboxysomes contains a small protein (~10 kD) (Pfam00936, known as CsoS1) which crystallizes as cyclic hexamers. Inspection of the Prochlorococcus genome reveals the CsoS1 gene (PMM0549 in MED4), and a 256 amino acid protein predicted to contain a BMC domain in its C-terminus. This larger protein (PMM0547 in MED4) and its genomic position are conserved in all of the marine Prochlorococcus.